1. Field of the Invention
The invention relates to a screw, more particularly to an improved waterproof screw fastener.
2. Description of the Related Art
Screw fasteners are used to join building materials such as plastic roofing boards. Referring to FIGS. 1 and 2, a conventional screw fastener (1) includes a head (11), a threaded shank (10) extending downwards from the head (11), and a sealing washer (12) sleeved onto the threaded shank (10) below the head (11). The head (11) has an upper hexagonal portion provided with six flat side faces (13). The sealing washer (12) is made of an elastic material. A wrench (20) having a bore with six inner faces (21) is used to hold the upper hexagonal portion of the head (11) and to turn the screw fastener (1) joining two separate members (22, 23). The sealing washer (12) is pressed by the head (11) in order to prevent water from seeping through the engaging holes of the members (22, 23). When the sealing washer (12) is pressed by the head (11), the periphery of the sealing washer (12) extends outwards and is not covered or protected by the head (11). The periphery of the sealing washer (12) is easily exposed to the elements, thereby easily and wearing the sealing washer (12) and shortening its effective service life. In addition, the screw fastener (1) does not have a good outward appearance.
The inner faces (21) of the wrench (20) gradually wear after a long period of use. The engagement between the flat side faces (13) of the head (11) and the inner faces (21) of the wrench (20) therefore becomes loose. Thus, when the wrench (20) is used to turn the screw fastener (1), the wrench (20) does not properly engage and easily detaches from the screw fastener (1).